Abstract

A new nonsulfonylurea oral hypoglycemic agent, JTT-608, has been reported to stimulate insulin release at elevated, but not low, glucose concentrations and consequently not to induce hypoglycemia in rats. Accordingly, this drug is potentially a safer antidiabetic agent than sulfonylureas. To explore the mechanisms underlying this glucose-dependent insulinotropism, the present study investigated the effects of JTT-608 on cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) and protein kinase A (PKA) activity in rat islet β-cells by microfluorometry using, respectively, fura-2 and a fluorescence PKA substrate, DR II. In the presence of glucose at normal and elevated concentrations (5.0–16.7 mM) JTT-608 (30–1000 μM) concentration dependently increased [Ca²⁺]_i in up to 88% of single β-cells, whereas at lower glucose concentrations (2.8 and 4.2 mM) it had little effect. The [Ca²⁺]_i responses were inhibited under Ca²⁺-free conditions and by nitrendipine, an L-type Ca²⁺ channel blocker. JTT-608 rapidly activated PKA and a PKA inhibitor, H89, inhibited [Ca²⁺]_iresponses to JTT-608. JTT-608 also stimulated insulin release from rat islets in a glucose- and Ca²⁺-dependent manner. The glucose-unresponsive β-cells, which failed to respond to 8.3 mM glucose with increases in [Ca²⁺]_i, were frequently recruited to [Ca²⁺]_i increases by JTT-608. JTT-608 also induced oscillations of [Ca²⁺]_i. Glucagon-like peptide-1(7-36)amide (GLP-1), pituitary adenylate cyclase-activating polypeptide (PACAP), and acetylcholine (ACh) enhanced the action of JTT-608 on [Ca²⁺]_i. In conclusion, JTT-608 evokes PKA-mediated Ca²⁺ influx and Ca²⁺ signaling in rat islet β-cells in a glucose-regulated manner, which may account for its glucose-dependent insulinotropism. JTT-608 and neurohormones may cooperatively activate islet β-cells under physiological conditions.